This invention pertains to optical communications systems, in general, and to an optical network and a method of operating an optical network, in particular.
As used herein, the term xe2x80x9coptical networkxe2x80x9d relates to any network that interconnects a plurality of nodes and conveys information between nodes with optical signals. The term xe2x80x9coptical communications systemxe2x80x9d as used herein refers to any system that utilizes optical signals to convey information between one node and one or more other nodes. An optical communications system may include one or more optical networks.
Telecommunications carriers began installing optical fiber cable about 15 years ago. At the time the optical fiber cables were installed, it was expected that the optical fiber infrastructure would provide communications systems and networks with ample capacity for the foreseeable future. However, the phenomenal growth of data traffic on the Internet has taxed the capabilities of the optical fiber infrastructure. In addition, new high bandwidth applications are being developed and are being made available for corporate applications. The result of this increased usage of the fiber infrastructure is serious network congestion and exhaustion of the fiber infrastructure. In the past, optical fiber systems relied on time division multiplexing to route traffic through a channel. Time division multiplexed systems add more capacity by time multiplexing signals onto an optical fiber. A disadvantage of time division multiplex systems is that data must be converted from light waves to electronic signals and then back to light. The system complexity is thereby increased.
As the demand for increasing traffic capacity continues, the limitations of existing optical networks and optical communications systems must be overcome. To do so, the capacity of the existing optical networks and optical communications systems needs to be increased. Capacity of existing optical infrastructure may be expanded by the laying of more fiber optic cable, for example. However, the cost of such expansion is prohibitive. Therefore a need exists for a cost-effective way to increase the capacity of existing optical infrastructure.
Wavelength Digital Multiplexing (WDM) and Dense Wavelength Digital Multiplexing (DWDM) are being used and/or proposed for use in long-haul telecom network applications for increasing the capacity of existing fiber optic networks. The advantage of both WDM and DWDM is that the conversion to electrical signals is not necessary. The devices that handle and switch system traffic process light and not electrical signals. WDM and DWDM would appear to many to be the solution to optical network limitations. In WDM, plural optical channels are carried over a single fiber optic, with each channel being assigned to a particular wavelength. By using optical amplifiers, multiple optical channels are directly amplified simultaneously thereby facilitating the use of WDM systems in long-haul optical networks. DWDM is a WDM system in which channel spacing is on the order of one nanometer or less. WDM and DWDM expand the capacity of an optical fiber by multiple wavelength channels into a single laser beam. Each wavelength is capable of carrying as much traffic as the original. Thus in one example set forth by Barry Greenberg in Special Report: new growth markets/emerging OEMs: lighting the way to a network-capacity solution, Electronic Buyer News, 04-19-1999, pp. 50, xe2x80x9cA fiber carrying four 2.5-Gbit/s DWDM channels, for example, has its capacity increased to 10 Gbit/s, without having to install additional fiber of use higher-speed transmission equipment.xe2x80x9d With WDM and DWDM, traffic passes from one node of the network to its destination in the form of light waves without conversion to electrical signals. DWDM and WDM will permit increase in the capacity of the fiber infrastructure. Systems with up to 128 and 240 DWDM channels have been proposed and/or are being built. However, DWDM and WDM are both limited by the non-linear cost increase as the network is expanded. In each instance, expansion beyond an incremental increase in traffic handling capacity may trigger significant investment in new optical fiber and equipment that is significantly in excess of the incremental increase in network capacity. In addition, DWDM based systems are not scaleable in expansion because equipment typically has to be replaced rather than merely added to. Existing implementations of both WDM and DWDM are too limited for solving the congestion problems of the existing optical infrastructure. The present systems are limited in the number of available channels. The slight increase in channel occupancy in such systems will present severe restrictions on the traffic handling capacity of the network. Additional difficulties with present implementations of DWDM and WDM technology include lack of flexibility; difficulty in handling packet switched information, non-linear optical effects and the already noted lack of incremental and scaleable upgrade capability.
It is therefor highly desirable to provide an optical communication system that has increased channel capacity. It is further desirable to provide an optical communication system that provides bandwidth based upon user demand. It is further desirable that any improved optical communication system is able to utilize the existing fiber optic infrastructure. Such a solution will prevent so called xe2x80x9cfiber exhaustxe2x80x9d. To effectively utilize the existing infrastructure in the face of the dramatic increases in traffic that will be encountered, it is highly desirable to increase channel capacity by a factor of 10 to 200 times that provided by DWDM to permit up to 20,000 channels to be served. It is also highly desirable that any improved optical communication system has a low cost per channel.
In a method of operating an optical communications system in accordance with the invention, a plurality of optical channels is provided and the optical channels are utilized for communications among a plurality of communications nodes. Each optical channel is determined by at least two of three optical signal characteristics. A first one of the optical signal characteristics is selected from a plurality of predetermined optical wavelengths. A second one of the optical signal characteristics is selected from a plurality of predetermined optical phases, and a third one of said optical signal characteristics is selected from a plurality of optical modulation frequencies.
In one embodiment in accordance with the principles of the invention, two of the optical signal characteristics are utilized to determine the optical channels. In this first embodiment of the invention, each channel is defined by one optical wavelength of a plurality of optical wavelengths and by one modulation frequency of a plurality of optical modulation frequencies.
In a second embodiment in accordance with the principles of the invention, one wavelength of a plurality of optical wavelengths, one frequency of a plurality of optical modulation frequencies, and one phase of a plurality of optical signal phases define each channel.
In a system in accordance with the principles of the invention, an optical network having a plurality of nodes, each node being coupled to the network, is provided with a laser source that serves as a reference to synchronize operation of the network. Still further in accordance with the principles of the invention, the reference laser optical output is distributed to all nodes of the network. In the illustrative embodiment of the invention, the reference laser output is distributed via a separate fiber optic path. By utilizing a distributed reference laser, a plurality of channels may be defined by.
In accordance with one aspect of the invention, the laser reference is used to generate a plurality of channels for communication paths through the network. In accordance with another aspect of the invention, the laser reference is a multiple wavelength laser.
Optical communication system apparatus and methods of operating an optical communications system in accordance with the invention may utilize existing optical fiber networks and provide significantly increased channel capacity. In accordance with one aspect of the invention the system apparatus provides for a plurality of communications channels and a processor unit receives requests for allocation of one or more channels from a node coupled to the optical communications system. The system apparatus dynamically allocates one or more channels selected from unused channels.
Still further in accordance with the invention, any node coupled to the communications system can be coupled to any other node via any unused channel. In accordance with one aspect of the invention, the selection of a channel is in accordance with a predetermined algorithm. One algorithm is such that the channel distance between the assigned channel and other channels in use is maximized. Another algorithm is such that cross channel interference between the assigned channel and channels in use is minimized.